With the ability to provide signal gain, double-hop elimination, IP multiplexing and mesh networking without a teleport, what impact can on-board processing have on satellite communications? On-board processing has the potential to bring video on-demand, capacity growth and convergence of ground and space networks, thus promising to revolutionize satellite as we know it.
But what exactly is on-board processing, and what are the characteristics of the payloads equipped with this technology? “Satellite architectures can be put into three categories: conventional bent-pipe radiofrequency systems, on-board processed bent-pipe systems and on-board digital processed systems,” says Jeff Snyder, group vice president, SkyTerra Communications Inc., a mobile satellite services operator that relies on ancillary terrestrial component (ATC) technology. “The first type is used in fixed service and direct broadcast satellites, the second type in systems such as SkyTerra satellites, while the last type is typical of systems such as the Iridium constellation.”
The differences between these systems are significant. On-board processed bent-pipe systems typically perform frequency channelization of signals by digitally processing them at baseband or near baseband. The resulting channelized signals can then be conditioned and processed prior to up-conversion for retransmission. However, the signals are not demodulated to their digital content. “The advantage of on-board processed bent-pipe systems over conventional bent-pipe systems is that relatively narrowband channels can be processed and the resulting channels can be switched between a large number of beams,” says Snyder. In other words, this architecture allows for the optimization of the satellite’s spectrum assignments, directing satellite resources where they are needed. This results in frequency reuse in narrowband communications networks.
On-board digital processed systems, on the other hand, take the capacity optimization process a step further. They process signals by demodulating and decoding the received waveforms to the digital packet or bit level. “This allows the system to have extremely flexible signal and information routing,” says Snyder. Depending on the capabilities of the on-board digital payload, the resulting individual data streams can even be decoded and recoded to match the channel characteristics of the intended downlink beam. These systems allow capacity reassignment in real-time at the packet level in response to the traffic loading. Furthermore, depending on the complexity of the digital payload, services such as packet replication and directed multicasting can also be implemented. That is to say that on-board digital processing transforms satellite payloads into “routers in the sky,” but that is not all — crucially, such systems also enable inter-satellite links, thus reducing the need for gateways in a global satellite network.
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